In order to authenticate and verify the originality of, and to prevent unauthorised duplication or alteration of documents such as banknotes, credit cards and the like, security devices are often incorporated. The security devices are designed to provide some proof of authenticity and deter copying. Despite the wide variety of techniques that are available, there is always a need for further techniques which can be applied to provide a security device.
A variety of techniques have been developed to conceal latent images within security documents and instruments. Perhaps the earliest such technique is the Watermark. In this approach, a latent image is provided on a paper substrate such that the image is invisible when the paper is viewed in reflection, but visible when it is viewed in transmission.
More recent means of concealing images for security applications include the technique known as “Scrambled Indicia” and described in analogue form in U.S. Pat. No. 3,937,565 and in a computerized, digital version in WO 97/20298. In the latter technique, the computer program effectively slices the image to be hidden into parallel slivers called “input slices”. These are then scrambled, generating a series of thinner “output slices” that are incorporated into an image in a form that is incoherent to the human eye. When viewed through a special device containing many microscopically small lenses, the original image is, however, reconstituted, thereby rendering the hidden image visible.
Scrambled images of this type may be incorporated into a visible background picture by adjusting the thickness of the features in the scrambled images.
WO 97/20298 also describes how the scrambled images may be routinely incorporated into a visible picture by a computer algorithm. An original image is digitised and separated into its cyan, magenta, yellow, and black components. One or more scrambled images are then incorporated into the cyan and magenta separations. These are substituted for the originals and the job is printed as normal.
A variety of patents also describe the concealment of latent images by “modulation” of the line- or dot patterns used to print images. In order to print an image, professional printers use a variety of so-called “screening” techniques. Some of these include round stochastic line and elliptical-screens. Examples of these screens are shown in U.S. Pat. No. 6,104,812. Essentially, the picture is broken up into a series of image elements, which are typically dots or lines of various shapes and combinations. These dots and lines are usually extremely small, being much smaller than the human eye can perceive. Thus, images printed using such screens appear to the eye to have a continuous tone or density.
Hidden images can be created by juxtaposing two apparently similar line or dot screens with one another. Processes in which an image is hidden by changing the position, shape, or orientation of the line elements used in printing screens are formally known as “line modulation”. Processes in which the dots in a printer's screens are deformed or moved to conceal an image are known as “dot modulation”.
It would be desirable to provide other techniques for concealing images and/or to provide decoders with improved security.